The proposed research focuses on the SERCA calcium pump and the NKA sodium pump. These are two of a class of proteins called Ion-motive ATPases, enzymes that use the energy of ATP to pump ions across cell membranes. Ion pumps play an important role in all cells, and disordered function of these enzymes is associated with human disease, including heart failure. The proposed research will use advanced fluorescence methods to measure the structures and motions of ion pumps in living cardiac muscle cells. We will test three major hypotheses: 1) Ion pumps undergo very large structure changes. 2) These large slow changes are regulated by fast protein dynamics. 3) Regulatory partners continually bind and unbind from the pump with fast kinetics. By measuring ATPase structures and motions in live cardiac cells, we will gain new insight into mechanisms that become disrupted in failing cardiac muscle.